This invention relates to a composition and method useful for treating metal surfaces to enhance bonding of the metal surfaces to other materials, and in particular to enhance bonding of metal surfaces.
Materials such as metals, polymers, and ceramics are bonded to one another, or bonded to coatings such as polymers, enamel, glass, ceramic, magnetic ferrite, or refractory materials. Examples of industrial applications include the bonding of structural metal or composite assemblies using polymeric adhesives, widely used in the aircraft industry, and increasingly used in the chemical engineering and automobile industry. For example, aluminum alloy components are adhesive bonded to one another to form structural aircraft components having reduced weights and which can be manufactured at lower costs. Other examples include powder painted aluminum components, and aluminum bonded to PTFE coatings to yield non-stick or low friction surfaces. In these applications the metal surfaces of the components are chemically treated prior to bonding to promote adherence and corrosion resistance of the adhesive-bonded joint or interface between the metal components. For example, a typical three layer adhesive-bonded joint between two aluminum alloy components comprises (1) an aluminum oxide layer on the aluminum component surface; (2) a primer layer on the oxide layer; and (3) an epoxy adhesive layer on the primer layer for bonding the aluminum components to one another.
The durability and corrosion resistance of the joint between the metal surface and the material bonded to the metal surface is particularly important in structural applications, such as aircraft structures, because these joints are exposed to a wide range of environmental conditions with extreme temperatures, high humidity, and highly corrosive marine environments. To avoid failure of the joint as well as to meet stringent commercial passenger and cargo aircraft standards, the adhesive-bonded joint of the structural component must withstand the harsh environmental conditions, and in particular resistance to corrosion and disbonding in humid salt laden environments, especially those resulting from sea spray or deicing materials. Failure of these joints often starts with diffusion of water through the adhesive followed by corrosion of the underlying metal structure. Thus it is desirable to have a method and composition useful for bonding metal surfaces that delays onset of corrosion and exhibits stability in aqueous and salt laden environments.
Conventional surface treatment processes have several disadvantages. Current pretreatment processes include anodizing the metal surface in a bath of: (i) chromic acid as disclosed in U.S. Pat. No. 4,690,736; sulfuric acid as disclosed in U.S. Pat. No. 4,624,752; (ii) phosphoric acid; (iii) oxalic acid; or (iv) a mixture of sulfuric and chromic acids. These processes form a partially hydrated oxide coating on the metal surface. The partially hydrated oxide coatings corrode in humid environments, for example aluminum oxide coatings on aluminum surfaces corrode to form aluminum hydroxide, in particular a mixture of boehmite (Al2O3H2O) and pseudo-boehmite (Al2O3H2O), which is mechanically weak and adheres poorly to the aluminum metal. Further hydration leads to formation of bayerite Al(OH)3 which results in disbanding of the joint. Thus, conventional anodizing processes can lead to hydration instability and failure of the joint.
Another disadvantage of chromate and phosphate based anodizing processes is that these processes typically use large amounts of water to neutralize the treated metal surfaces, and to rinse off the corrosive acids used for anodization of the metal surface. Disposal of the phosphate or chromate containing waste water is expensive and can be environmentally hazardous. Commercial anodizing processes also require large amounts of electricity to sustain an anodizing current in the anodizing bath, particularly for large metal components, and require expensive equipment such as large anodizing and rinsing tanks, automatic systems for transferring the metal component from the anodizing tank to the rinsing tank, and sizable electrical power supplies. Thus it is desirable to have a bonding composition and method that does not use excessive amounts of water or electricity, and that can be used without large capital outlays for expensive equipment.
Another disadvantage of conventional treatment processes is their narrow processing window. Deviation from the processing window can result in poor bonding. For example, in phosphoric acid anodizing processes, if the metal component is not removed from the phosphoric acid bath immediately after the anodization current is turned off, the anodized oxide coating formed on the metal component can be rapidly dissolved by the corrosive chromic or phosphoric acid bath, resulting in a loosely bonded oxide coating. Thus, it is desirable to have a surface treatment process that provides a relatively large processing window to allow flexible production schedules while minimizing failure of the bonded joint.
Another significant disadvantage of conventional surface treatment processes arises from their use of highly toxic and hazardous chemicals, such as hexavalent chromium compounds. Chromic compound rinses are used to seal phosphoric acid treated metal surfaces to provide adequate corrosion resistance. Disposal of the waste chromic byproducts, and the large amount of metal sludge dissolved in the acid, has become increasingly expensive in view of stringent environmental regulations and standards. Thus many conventional surface treatment processes are being gradually phased out because of the environmental regulations. Therefore, it is also desirable to have a non-toxic surface treatment process that is substantially environmentally benign.
The present invention satisfies these needs. In one aspect, the present invention provides a bonded structure comprising a first component having a first surface bonded to a second surface of a second component by a bond layer comprising an adhesion layer, wherein the adhesion layer comprises a cured liquid composition comprising water, a metal alkoxide of the formula M(OR)x where M is a metal and R is an alkyl group, an organoalkoxysilane, and an acid. Preferably, the molar ratio of metal alkoxide:organoalkoxysilane:acid is selected so that the adhesion layer formed upon curing the liquid composition forms a strong and corrosion-resistant joint between them. Preferably, the organoalkoxysilane comprises silane functional components or groups that can react and bond to the two surfaces. The first and second surfaces are preferably metal surfaces.
In another preferred embodiment, the present invention provides a bonded structure comprising a first component having a first surface bonded to a second surface of a second component by a bond layer comprising an adhesion layer, wherein the adhesion layer comprises a cured liquid composition comprising water, metal alkoxide of formula M(OR)x where M is metal and R is an alkyl group, a single organoalkoxysilane, and an acid. Preferably, the molar ratio of metal alkoxide:organoalkoxysilane:acid is preferably from about 1:0.5:0.1 to about 1:15:0.8, and more preferably from about 1:1:0.2 to about 1:8:0.5.